Grazing of the mixotrophic dinoflagellate Ceratium furca on ciliate populations

Dinoflagellate mixotrophy may be among the more important microbial pathways in enriched coastal regions, yet relatively little is known about feeding rates of these mixotrophs and their impact on prey populations. We employed a method using fluorescent microspheres to label ciliates and follow their ingestion by mixotrophic dinoflagellates of the genus Ceratium, principally C. furca. Experiments with natural water were conducted in Chesapeake Bay, the east coast of Florida (Indian River Lagoon), and the Gulf Stream east of the Indian River. Ingestion rates of C. furca were similar in Chesapeake Bay and Florida, ranging from 0 to 0.11 prey h-1. Clearance rates decreased with increasing prey density. In Chesapeake Bay, C. furca removed up to 220 % of Strobilidium spp. standing stock daily. Impact on total ciliate standing stock was lower (less than 62 % removed daily). Laboratory feeding experiments using Strobilidium spp. as prey showed comparable trends. Ingestion rates increased up to 0.16 prey h-1 with increasing prey density, and saturated above 60 Strobilidium ml-1. Clearance rates were similar to those of the field experiments. This study indicates that grazing by C. furca can at times have a significant impact on Strobilidium spp. and even total ciliate standing stock.